Directional wireless system employing pulses



Dec. 17, 1940. W. s. PERCIVAL 2,225,524

DIRECTIONAL WIRELESS SYSTEM EMPLOYING PULSES Filed NOV. 16, 1938 /NVE/VTO William pencrPercivaZ Patented Dec. 17, 1940 DIRECTINAL WIRELESS SYSTEM EMPLOYING PULSES William Spencer Percival, Ealing, London, Ilngland, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application November 16, 1938, Serial No. 240,602 In Great Britain November 16, 1937 Claims.

The present invention relates to wireless communication and direction finding systems in which the signals transmitted are in the form of pulses as distinct from a continuous wave trans- 5 mission and relates also to wireless communication systems in which a carrier employing continuous waves has a pulse transmission superimposed thereon. The invention is concerned with the prevention of interference with reception of the signals, and with the application of certain interference suppression technique to provide receiving apparatus of a highly directional type.

There has previously been described, in the specifications of British Patents Nos. 467,263 and 468,994, a system for suppressing short *pulses which interfere with the reception of carrier wave transmission. The system described in these specifications is of the type having an auxiliary receiver designed to discriminate in favour of interference and which serves to switch oil? the main receiver for short periods corresponding to each pulse of interference which exceeds a certain amplitude. It has been pointed out in the 25 latter specification that the auxiliary receiver may be connected to a separate aerial located and orientated in such a way as to respond less to the Wanted signal than does the main aerial and to be more sensitive to interference than the main aerial.

According to the present invention a system for the transmission of short pulses comprises means for the reception of the pulses in which pulses or interfering signals other than a specified pulse 35 transmission are prevented from being received by a main channel by means of an auxiliary channel.

The invention will now be described by way of example with reference to the accompanying drawing which illustrates a receiving circuit according to the present invention.

Pulses sent out from different sources, or sets of pulses sent out from the same source are differentiated and selectively amplified by the means illustrated.

Terminal I1 is the inputfterminal to a main channel and terminal I2 is the input terminal to an auxiliary channel. These terminals are connected to the same or different receiving aerials I, 3 respectively. The main channel is relatively `sharply tuned to the frequency of the carrier it is desired to receive, that is to say it has a relatively narrow pass band, whilst the auxiliary channel is relatively flatly tuned.

The auxiliary channel comprises a circuit LLC;

(Cl. o-20) tuned to the frequency of the desired carrier or to some other frequency. A valve Vi, acts as a mixer valve for'mixing the oscillations from circuit Li C7 with local oscillations fed from an oscillator V2 through a condenser Cn. In one 5 example circuit L1 Ci is tuned to a frequency between 15 and 45 megacycles per second and the oscillatorv V2 serves to produce an intermediate frequency of 4.8 megacycles per second.

The valves V3, V4, V5 and Ve are arranged to 10 amplify at this intermediate frequency. Tuning of the intermediate frequency stages is eiected with the aid of variable inductances L4 and L5 which are damped by means of resistances R1 i and R2. The tuning 'of thevarious stages may 15 be staggered so that with the damping resistances the desired breadth of tuning is obtained. 'I'he pass-band for the intermediate frequency amplifier may be 150 kilocycles per second.

The output of the intermediate frequency am- 20 plii'ler is coupled to a diode detector V1 the output of which passes through a filter to the inner grid of a heptode valve Vs which constitutes the rst valve of the main channel. The lter comprises shunt condensers C5, Cs and C1 and series 25 elements, each comprising a resistance and an inductance R3 Lr, Rr La and R4 La. The filter is so designed as to remove the carrier frequency of say 4.8 megacycles per second and to pass without substantial attenuation the modulation fren quencies passed by the intermediate frequency amplifier, that is frequencies below about 75 kilocycles per second. It is also designed so as to avoid resonances which-might cause transients to give rise to variations of an oscillatory character on the inner grid of valve Va. The output of valve Vs is taken fromlterminals P to a receiver 2 of normal/type which with the valve Va constitutes the main channel and passes a nar-l rower band of frequencies than the intermediate frequency amplier V3, V4, Vs and Ve.

A suitable positive bias is applied to the cathode of valve Va relative to its control grids by means of a resistance Re shunted by a by-pass/ condenser Ca. A suitable voltage may be impressed between terminals V to make the cathode of the diode Vr somewhat positive relatively to the anode thereof and thus to back-off the diode and prevent it passing current for signals below a predetermined amplitude.

The input terminal I1 of the main channel is connected to the outer control grid ofthe valve`Va through a band-pass delay network L10 C3 L11 C4 which is designed to give to the signals passed therethrough a delay equal to or slightly greater 55 auxiliary channel comprises, in this example, an

`frequency as the main channel or to a diii'erent additional intermediate frequency stage including a valve Vo and a tuned circuit L1: Ra connected to the output or valve Ve. The valve Va is of such type that when given a low screengrid voltage it operates as a limiter, partly by grid current damping and partly by anode current cut-ofi'. The output of this valve Va is Ied lto a diode detector V and the output of the latter is fed from a lter F to the grids of one or more of `the valves of the intermediate frequency amplifier Va, V4, Vs, Vo.

The auxiliary channel is connected at terminal Ia to an auxiliary aerial 3 which is either` antidirectional to a specified pulse transmission or which is responsive to interfering signals or pulses arriving in the same direction as the specified pulse transmission but having different states of polarisation.

The operation ofthe circuit above described is 'as follows:

Whether the circuit L1 Ci is tuned to the same frequency, both terminals I1 and Ia receive the same transient interference and that in the auxiliary channel is detected at V7, if above the minimum amplitude (determined vby the voltage at V) to which the detector V'z is responsive, and is applied as a negative pulse to the inner control grid oi valve Va. It is arranged by a suitable choice of valve and bias `for Va that even a small transient interference signal is capable of cutting oil? the valve Va by arresting the flow oi.' electrons therein. Since the delay in the main -channel is equal to or' slightly greater than ,that

in `the auxiliary channel, the amplication of valve Va and hence that of the main channel, will have been reduced substantially to zero by the time a transient reaches the outer control grid of valve Vs from terminal I1.

The valve V9, by acting as a limiter, ensures that signals above a relatively low amplitude have no further effect upon the A. V. C. voltage developed. Moreover, since the energy content of transients of the same amplitude as a continuous Wave is much smaller than the energy content of the continuous wave, transients of the maximum amplitude that can pass the limiter/have but little influence on the A. V. C. voltage developed. Thus the A. V. C. voltage is mainly dependent upon the continuous waves and it can be arranged that the amplication of the intermediate frequency amplier V3, V4, V5, Vs varies as the amplitude of continuous waves varies in such a way as to maintain the output of continuous waves from the valve Ve substantially constant and .small and that the action is substantially unaffected even by transients of very large amplitude.

The aerial I supplying the main channel may be arranged so as to be more sensitive than the other aerial to the desired signal 5. Thelatter aerial on the other hand may be made relatively more sensitive to transient interference by suitable location or orientation thereof. For example, the aerial of the auxiliary channel may be a length of wire taken close to a source of interference 1, the effect of which on the main channel it is required to eliminate.

The auxiliary aerial may also be made sensitive to pulse transmissions arriving from directions other than from the direction of the required signal pulse or to pulses polarised in a nazafata;

accurately determined or pulses reflected by an aeroplane may be utilised to give an .indication oi the position of the aeroplane. As a further example, obstacles at sea may be located by applying short pulses to the'water and observing the directions from which pulses are reflected back again. Reflections fromV directions other than I'those which are being observed, and any other interfering pulses, -will be suppressed as previously described. It is of course assumed that the unwanted pulses do not, in,general, arrive at the Lsame instant as the wanted pulses, so that the main receiver is not disabled at the moment of reception of a wanted pulse. j

When there exist a plurality of pulses or intertering signals which it is required to prevent the main channel from receiving, then it may be necessary to employ a plurality oi auxiliary channels.

The auxiliary channel together with its associated aerial system may be made to respond appreciably to signals which emanate from directions making but small angles with the required signal so that the response of the main channel will be a maximum to the required signals, which maximum response decreases rapidly due to the actionof the auxiliary channel. By this means there is furnished a direction iinding system which operates on the maximum value of the required signal, and which maximum decreases rapidly on either side of the direction of the maximum signals.

By suitably biassing 4 a valve or valves of the receiver the latter may be made insensitive to signals below a predetermined amplitude value. Small signals and background noise may thus be suppressed, and with this arrangement the pulses used for transmitting may be given an increased time interval for each pulse, as the effect on the receiver depends upon the energy content of the whole pulse. Background noise which may be of a continuous character but have only a small amplitude will be eliminated by such biassing of a valve or valves of the receiver, the pulses being above such eliminated amplitude level and thus are unaffected thereby.

Apparatus according to the invention may be applied to receive, without mutual interference, one at a time, a number of puise transmissions from different stations provided that these transmissions all diier in direction or plane of polarisation, and provided that the gaps between pulses are large compared with the width of each pulse so that the probability of pulses from different stations coinciding in time is reasonably small. The interval or gap between the pulses determines the frequency of the pulses and this frequency may be modulated as required.

A transmission system according to the present invention may utilize a continuous wave transmission and a system of pulses which both employ the same frequency band and thus two sets of messages may be transmitted,.these being dealt with at the receiving end by two receivers, of which one receives only the continuous waves, while another receiver receives only the system of pulses.

Further, a plurality of pulse transmissions may be generated at a transmitter, which pulse transmissions are polarized in different planes and preferably arranged so that'the separate pulse transiiissions do not coincide in time. The transmissions may be separated out at the receiving station by receiving apparatus of the kind described above in which the main and auxiliary aerials can be adjusted to discriminate between f signals arriving from the same direction but having different planes of polarization.

It may be desired to examine the shape or exact time of arrival of a signal pulse for certain purposes-and a dimculty may arise here in the following way. The auxiliary receiver, particularly if it has a large ampliiication, may, due to noise and small disturbances, produce a varying control bias which varies the gain of the main receiver without actually switching it oil, and this variation of gain will distort the wave form of the amplified pulses.

According to a'further feature of the invention this difilculty is overcome lby interposing a trigger device 6, such as a gas filled valve or a blocking oscillator, between the output of the auxiliary suppressor receiver and the point at which the main channel is switched oil, so that the main channel must be either normally operative or substantially inoperative, but cannot be continuously modulated by small variations in the output of the auxiliary receiver. Alternatively, the trigger device may be used to short circuit the main channel. The recovery period of the trigger device must be reasonably short to avoid leaving the main receiver switched oil? for too lengthy periods. Under these conditions the waveform of the pulses will be preserved and may be examined, for example, on a cathode ray tube, the image possibly appearing faint if the main receiver is being very'frequently switched off. I claim:

1. A radio receiving system including in combination a main receiving channel having a narrow response band and including a control tube, an auxiliary channel having a broad response band and including an ampliiier and a rectifier, a directional antenna connected to said nrst channel, a second antenna connected to said auxiliary channel, means for preventing said rectiner from responding to signals below a pre- 50 determined level and permitting response to signals exceeding said level, means connecting said rectier to said control tube so that rectified Vcurrents may be applied to said control tube to diminish the response o1' said main channel to said excessive signals, and a second ampliiler connected to said first-named ampliiier, a rectiner connected to said second ampliner, and

. means connecting said last-named rectifier to a directional antenna connected to said firstchannel, a second antenna connected to said auxiliary channel, means for preventing said rectiiler from responding to signals below a predetermined level and permitting response to signals exceeding said level, means connecting said rectifier to said control 'tube so that rectified currents may be applied to said control tube to diminish the response of said main channel to said excessive signals, and a trigger device interposed between said rectier and said oontrol tube so that said main channel is substantially insensitive to small variations in the output of said auxiliary channel.

3. A radio receiving system including in combination a main receiving channel having a narrow response band and including a control tube, an auxiliary channel having a broad response band and including an amplier and a rectier, a directional antenna connected to said rst channel, a second antenna connected to said auxiliary channel, means for preventing said rectier from responding to signals below a predetermined level and permitting response to signals exceeding said level, means connecting said rectifier to said control tube so that rectied currents may be applied to said control tube to diminish the response of said main channel to said excessive signals, and means for preventing said main .channel from responding to signals below a predetermined amplitude value.

4. A radio receiving system including in combination a main receiving channel including a delay circuit and a tube for controlling the response of said channel, an auxiliary channel including a rectifier, an antenna connected to said flrst channel, a second antenna connected to said auxiliary channel, means for preventing'said rectifier from responding to signals below a predetermined amplitude value and permitting rectiiication of signals of amplitude exceeding said v value, means connecting said control tube and said rectiiier to diminish the response of said main channel to said signals of amplitude exceeding said predetermined value, and means for limiting the response of said main channel to signals of amplitude exceeding a second predetermined value.

5. A radio receiving system including in cornbination a main receiving channel including a delay circuit and a tube for controlling the response of said channel, an auxiliary channel including an amplifier and a rectifier, a directional antenna connected to said rst channel, a second antenna connected to said auxiliary channel, means for preventing said rectifier from responding to signals below a predetermined amplitude value and permitting rectication of signals of amplitude exceeding said value, mean-s connecting said control tube and said rectifier to f diminish the response of said main channel to said signals of amplitude exceeding said predetermined value, a second amplifier connected t'o said first-named amplifier, a second rectier connected to said second amplifier, and means connecting said second rectifier to said first amplifier to control automatically the sensitivity of said first amplier.

WILLIAM SPENCER PERCIVAL. 

